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The correct A20 masking is done if paging is enabled (protected mode) but it seems to have been forgotten in real mode. For example from the AMD64 APM Vol. 2 section 1.2.4: > If the sum of the segment base and effective address carries over into bit 20, > that bit can be optionally truncated to mimic the 20-bit address wrapping of the > 8086 processor by using the A20M# input signal to mask the A20 address bit. Most BIOSes will enable the A20 line on boot, but I found by disabling the A20 line afterwards, the correct wrapping wasn't taking place. `handle_mmu_fault' in target/i386/tcg/sysemu/excp_helper.c seems to be the culprit. In real mode, it fills the TLB with the raw unmasked address. However, for the protected mode, the `mmu_translate' function does the correct A20 masking. The fix then should be to just apply the A20 mask in the first branch of the if statement. Signed-off-by: Stephen Michael Jothen <sjothen@gmail.com> Message-Id: <Yo5MUMSz80jXtvt9@air-old.local> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> |
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